MIPS: Implement __read_mostly

Just do what everyone else is doing by placing __read_mostly things in
the .data.read_mostly section.

mips_io_port_base can not be read-only (const) and writable
(__read_mostly) at the same time.  One of them has to go, so I chose
to eliminate the __read_mostly.  It will still get stuck in a portion
of memory that is not adjacent to things that are written, and thus
not be on a dirty cache line, for whatever that is worth.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/1702/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>